The invention relates in general to gas detectors and in particular to a new and useful apparatus and method for measuring gaseous or vaporous media, where the variation or the rate of variation of the electrical resistance in a heated phthalocyanine containing sensor layer is used as a measuring quantity.
From German OS No. 28 09 873 is known a method for determining gaseous or vaporous media contained in the air, where the variation of the electrical resistance of a semiconductor selected from the series of the porphyrines, particularly phthalocyanine, is determined as a measuring quantity. The phthalocyanines can contain a metallic element, e.g. iron, nickel, cobalt, copper or manganese. As a sensor is used a coated contacted ceramic carrier which is connected to a device for measuring the resistance. As a resistance blank value of a sensor consisting of copper-phthalocyanine is indicated a value of 1.5.times.10.sup.6 ohm. The absorbed gases lead to a reduction of the resistance, whereby the differential of the variation is used as a measuring quantity. The absorbed gases can be desorbed again by heat treatment. There is no mention of the usability for detecting anesthetic gases in room air in the ppm-range. Besides, the sensitivity of the above described sensor is much too low for such a test.
British Pat. A No. 20 02 907 provided a semiconductor oxide film as a gas sensor, which is specifically suitable for detecting combustible and reducing gases and vapors, but not for anesthetic gases. The semiconductor layer is covered by an aluminum oxide film which carries a catalyst layer. The known embodiment permits no temperature differences between the semi-conductive oxide film and the catalyst layer.
German Pat. No. 1,271,430 and AS No. 1,299,141 describe devices for the continuous determination of the content of organic anesthetic vapors in the breathing gas of a patient, where the portion of the anesthetic causes an elongation of a strip of swelling silicone rubber, which is evaluated to indicate the measuring quantity. But these devices make it possible only to indicate the relatively high portion of the anesthetic gas in the breathing gas of a patient (about 1%) but not the concentration of this anesthetic gas in the ambient room air, which is by several orders of magnitude lower. As admissible is considered here a limiting value of only 2 ppm.
The state of the art also comprises the EMMA apparatus, described in the EMMA brochure of June 1980 by Engstroem Medical AG, which measures individual anesthetic gases, like halothane, enflurane, methoxyflurane and isoflurane. An oscillating quartz coated with silicon oil as a sensor is used. The gas absorption of the oil layer results in a frequency variation of the oscillating quartz, which can be determined in an electrical circuit as a measure of the anesthetic gas portion. The apparatus is only suitable for monitoring a relatively high anesthetic gas portion in the breathing gas of the patient, its sensitivity does not suffice to monitor the much lower room air portions.
For measuring the anesthetic gas halothane in the room air is known a testing method described in German AS No. 2,830,781 where a color change appears above a given limiting value in the test tube turned to pyrolytically obtained free halogen. The test tube permits monitoring of the room air portion of halothane by random tests with a relatively high sensitivity in the range of 1 ppm.